One typical example of conventional combustion pressure sensors for detecting a combustion pressure in a combustion chamber of an internal combustion engine is disclosed in Japanese Patent Application Publication No. 2015-152274, referred to as patent document 1.
The combustion pressure sensor disclosed in patent document 1 includes a housing whose tip end abuts on a reduced-diameter portion of a plug fitting hole of an internal combustion engine. An external screw mounted to a base end of the housing, which is opposite to the tip end, is threadably engaged with an internal screw formed to the plug fitting hole of the engine, so that the combustion pressure sensor is fitted in the plug fitting hole.
In the combustion pressure sensor, a compressive stress applied to the engine head based on the combustion pressure in the combustion chamber is applied as a load to the housing fitted in the plug fitting hole. The housing is therefore subjected to the compressive stress, so that the compressive stress is applied as a load to a load transfer member installed in the housing. This results in the load being transferred to a piezoelectric element held by a sensor holder, i.e. a sensor housing. This causes the piezoelectric element to measure the combustion pressure in the combustion chamber according to the load applied to the piezoelectric element.
Each of the load transfer member sensor holder is comprised of a hollow cylindrical member. The load transfer member has an annular tip end formed with an annular contact surface contacting with the housing, and an annular base end opposite to the annular tip end; the annular base end is formed with an annular contact surface contacting with an annular contact surface of the sensor holder.
Another typical example of such conventional combustion pressure sensors is disclosed in Japanese Patent Application Publication No. 2005-16984, referred to as patent document 2.
The combustion pressure sensor disclosed in patent document 2 includes a hollow housing with a tip end and a base end opposite to the tip end, and a diaphragm head having a diaphragm portion installed in the tip end of the hollow housing; the diaphragm head is subjected to pressure variations in the combustion chamber. The combustion pressure sensor also includes a piezoelectric stack, which is comprised of the set of stacked piezoelectric elements sandwiched and pressurized by first and second electrodes. The configuration of the combustion pressure sensor disclosed in patent document 2 enables the piezoelectric stack to detect variations of the pressure applied to the piezoelectric stack, thus detecting the combustion pressure in the combustion chamber based on the variations of the pressure applied to the piezoelectric stack.
In the combustion pressure sensor disclosed in patent document 2, a cylindrical inner body is installed in the housing, and provided at the base end side of the second electrode via an insulating ring. The cylindrical inner body includes a first cylindrical inner body and a second cylindrical inner body. The first cylindrical inner body has a tip end and a base end opposite thereto. The first cylindrical inner body has formed a conical concave surface at the base end, and the second cylindrical inner body has formed a spherical convex surface at the tip end. An external screw of the second cylindrical inner body is threadably engaged with an internal screw of the housing, and the conical concave surface of the base end of the first cylindrical inner body and the spherical convex surface of the tip end of the second inner body abut on each other. This results in the first cylindrical inner body and the second cylindrical inner body being in alignment with each other. This alignment of the first and second cylindrical inner bodies prevents an unbalanced load from being applied to the piezoelectric stack when the diaphragm head is subjected to pressure variations in the combustion chamber.